DESCRIPTION: (Applicant's Abstract) Despite major advances, the success of organ transplantation is currently limited by high rates of late graft loss due to chronic rejection, as well as high rates of malignancy and opportunistic infections, and other toxic side effects associated with chronic non-specific immunosuppressive drug therapy. The induction of specific transplantation tolerance would overcome all of these limitations. Mixed hematopoietic chimerism induces a reliable and robust form of donor-specific tolerance. The goal of this project has been to develop an approach to inducing mixed hematopoietic chimerism across MHC barriers with sufficiently high reliability and low toxicity that its use for the induction of organ allograft tolerance in humans would be justified. In the current funded period, the applicant has developed approaches, involving high dose stem cell transplantation and costimulatory blockade, that eliminate the requirement for recipient irradiation and T cell depletion in order to achieve mixed chimerism and long-term central, deletional tolerance. To advance the extension of these developments to the clinical arena, a major goal in this competing renewal application will be to understand the mechanisms involved in overcoming alloresistance and inducing tolerance in these models, so that 100 percent reliability can be achieved by the most stringent test of skin grafting. Specifically, the applicant aims to: 1) Determine the reason for variability in the outcome of BMT with costimulatory blockade and devise approaches to preventing failure of allogeneic marrow engraftment; 2) Determine the mechanism(s) of peripheral tolerance induced with costimulatory blockade and allogeneic BMT; 3) Evaluate the role of the thymus in tolerance induction and the timing with which intrathymic deletional tolerance is induced in mixed chimeras prepared with costimulatory blockade. The understanding obtained by achievement of these goals will be essential to our ability to reliably achieve marrow engraftment without toxicity in patients with non-malignant diseases, including those in need of organ transplants and those with hematologic disorders such as hemoglobinopathies.